1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically to a transistor that includes a zener diode that prevents surge destruction and to a manufacturing method therefor.
2. Description of the Related Art
A transistor of the past which included a zener diode, is shown in FIG. 11, in which the reference numeral 1 denotes an n.sup.+ type semiconductor substrate made of silicon, 2 is an n.sup.- type silicon region, 3 is a p-type base region, 4 is an n.sup.+ type collector surface region, 5 is an n.sup.+ type emitter region, 6 is an emitter electrode, 7 is a base electrode, and 8 is an oxide film.
In the above device, a zener diode is formed by the p-type base region 3 as an anode and the n.sup.+ type collector surface region 4 as a cathode, and an NPN transistor is formed by the n.sup.+ type emitter region 5, the p-type base region 3, and the n-type collector region 2.
In the above-described semiconductor device, by applying a positive voltage to the collector region 2 and a negative voltage to the base electrode 7, there arises the problem of a lateral broadening of a depletion layer formed by the influence of free ions in the oxide film 8 at the surface of the high-concentration collector surface region 4 and free ions within the outer resin over the high-concentration collector surface region, this in turn influencing the breakdown voltage (zener voltage) of the zener diode. To reduce the influence of the free ions, if the high-concentration collector surface region is covered at the base electrode by an oxide film, there is the problem of instability of the withstand voltage, because of the relationship between the base electrode and the extension length toward the high-concentration collector surface region.